Light-emitting diodes (referred to hereinafter as LEDs) represent one of the most popular light-emitting devices today. Due to the small form factor, LEDs are widely used in many electronic systems. One of the applications is in electronic display systems. Electronic display systems are usually large-scale display systems found in stadiums, discotheques, electronic traffic sign displays and infotainment boards along streets. The electronic display systems may be configured to display text, graphics, images or videos containing information or entertainment contents. Most display systems comprise hundreds or thousands of light-emitting devices arranged in a two dimensional plane for example in a matrix. The light-emitting devices in the electronic display system can be a white LED or a tri-color RGB LED. Each light-emitting device may represent a pixel in the display but in some occasions, a group of single colored light-emitting devices may represent one pixel.
In large-scale electronic display systems, the light-emitting devices may not be surface mount type, but through-hole type having a plurality of leads. The substrate is usually a printed circuit board (referred to hereinafter as PCB) having through-holes to accommodate the leads of light-emitting devices. The assembly of such electronic system usually involves inserting the leads of the light-emitting devices into corresponding through-holes, followed by soldering.
An embodiment of such prior art electronic system is shown in FIG. 1. FIG. 1 illustrates a cross-sectional view of a light-emitting device 105 soldered on a substrate 160 of a prior art electronic system 100. The light-emitting device 105 further comprises a plurality of leads 110, a light source die 130, and a body 140 encapsulating the light source die 130 and a portion of the leads 110. A portion of one of the leads 110 may be configured to define a reflector cup 120. The reflector cup 120 may be adapted to hold the light source die 130. The substrate 160 comprises a plurality of through-holes 165, a top surface 161 and a bottom surface 162.
The light-emitting device 105 attached on the top surface 161 by means of inserting the leads 110 of the light-emitting device 105 into the through-holes 165. The leads 110 are inserted into the through-hole 165 from the top surface 161 and a portion of the leads 110 protruded from the bottom surface 162. Soldering is usually done at the bottom surface 162. As shown in FIG. 1, the leads 110 are fixed to the substrate 160 by solder joints 170a and 170b. A complete solder joint 170a covers the entire through-hole 165 and establish reliable electrical connections. However, air may be trapped inside the through-hole 165. During the soldering process, the air inside the through-hole 165 may be heated up due to the thermal expansion. Consequently, the solder joint 170b may become defective having blowhole 195. The blowhole 195 serves as an air escape route when the air in the through-hole 165 goes thermal expansion.
The blowholes 195 may not be desirable. Without proper solder joint 170a, electrical connections may be intermittent. In addition, when the electronic system 100 is placed outdoor, further extreme external conditions such as high temperature and high humidity may stress the electronic system 100 further causing the solder joint 170b to break down. For outdoor uses, the problem become more pertinent as the operating condition may fluctuates substantially. For example, during daytime, the operating temperature may be 140° C. but during nighttime, the temperature may drop below −20° C.